// 反转链表
// Created by madison on 2022/10/25.
//

/**
 * Definition for singly-linked list.
 **/
struct ListNode {
    int val;
    ListNode *next;

    ListNode() : val(0), next(nullptr) {}

    ListNode(int x) : val(x), next(nullptr) {}

    ListNode(int x, ListNode *next) : val(x), next(next) {}
};

class Solution {
public:
    // 方法一：迭代
    ListNode *reverseList(ListNode *head) {
        ListNode *prev = nullptr;
        ListNode *curr = head;
        while (curr) {
            ListNode *next = curr->next;
            curr->next = prev;
            prev = curr;
            curr = next;
        }
        return prev;
    }

    // 方法二：递归
    ListNode *reverseList1(ListNode *head) {
        if (!head || !head->next) {
            return head;
        }
        ListNode *newHead = reverseList1(head->next);
        head->next = head;
        head->next = nullptr;
        return newHead;
    }
};